Pitch in a papermaking system can be simply defined as the sticky, resinous material that is released from wood during the pulping process. In paper mill process waters, pitch exists as an unstable, colloidal dispersion of hydrophobic particles. Under the conditions often encountered in a papermaking system, such as hydrodynamic and mechanical shear forces, abrupt pH and temperature changes and exposure to water hardness ions and inorganic scale deposits, colloidal pitch particles tend to agglomerate and deposit on paper machine surfaces.
Pitch deposits often lead to quality defects in the finished paper product, shortened equipment life, impaired system operation, paper machine downtime and, ultimately, lost profits for the mill. These problems are magnified when a paper mill "closes up" its process water system, as many mills have already done for conservation and environmental reasons, thus eliminating many potential exit points for pitch in the system. A closed, recirculating papermaking process water system only has a limited holding capacity for hydrophobic materials like pitch. Unless these pitch particles are continuously removed from the system in a controlled manner, spontaneous system purges can occur which lead to pitch deposits and runability problems. Thus, the control of pitch deposition in a papermaking system is a priority for many papermakers.
A number of pitch deposit control methods are used in the paper industry. For example, optimizing the performance of the pulp washing stages (e.g., kraft brown stock washers and bleach plant extraction stages) through the application of pitch dispersants and defoamers or washaids to these stages is a control option for many mills. The removal of pitch through these viable exit points is especially important in closed papermaking systems. The use of pitch adsorbants such as talc are often employed; however, unless the talc/pitch particles are effectively retained in the paper sheet, talc can end up contributing to, rather than solving, the pitch deposit problem.
Alum is a widely used pitch control agent for acid papermaking systems. It acts by attaching pitch particles to fibers in a manner analogous to the setting of rosin size. Cationic coagulants promote the attachment of the anionically charged, colloidal pitch particles to fibers and fines through a charge neutralization mechanism. The advantage to using cationic coagulants and alum for pitch control is that pitch is removed from the system in the form of microscopic particles dispersed among the fibers in the finished paper product. Unlike alum, a polymer's cationic charge is not necessarily dependent on the pH of the system, thus cationic polymers can be used effectively in neutral and alkaline paper machines. In addition, cationic polymers remain soluble under normal alkaline papermaking conditions while alum can form insoluble aluminum hydroxide.
It is commonly thought that cationic polymer retention aids act purely by charge neutralization to allow the anionic pitch to deposit on the anionic wood fiber instead of the hydrophobic plastic surface of the headbox and other papermaking machine parts. Pitch is typically formed from fatty acids, sterols, fatty alcohols, alkylesters, and fatty triglycerides. Pitch is released from wood by both chemical and mechanical processing. Colloidal pitch is generated during chemical pulping, during refining and beating operations, and in minor amounts at various stages of pulp bleaching.
Pitch is considered as a dilute oil-in-water emulsion, stabilized in part by the surfactants (e.g., fatty and resin acids in their ionized forms) generated during the papermaking process. Chemicals added during processing can also contribute to the stabilization of this emulsion. Representative particle size distribution of the colloidal pitch in unbleached kraft, sulfite and groundwood pulps is found to be in the range of 0.2 to 2 .mu.m. The total amount of colloidal pitch in suspension has been reported to lie in the range of 5-70 ppm (by volume) for pulps at 1% fiber consistency.
Pitch is harmless if removed properly from the papermaking system by either washing the pitch to the sewer, or removing it with the sheet as microscopic particles. However, if the pitch is allowed to concentrate in a system, agglomeration begins, sheet defects and build-up on equipment ensue, and lost or downgraded production are the end result of not properly managing pitch that enters the papermaking system with the pulp.
One concern in the manufacture of paper is the removal of depositable pitch. Several mechanisms regarding the prevention of pitch deposition control have been suggested in the past. Three mechanisms that are commonly proposed to explain the phenomena of pitch deposition control are outlined below:
(1) a cationic polymer used for pitch control adsorbs onto the anionic pitch particles, and these "stabilized" pitch particles become "fixed" onto the fibers, thus reducing the concentration of "free" pitch particles in the process water;
(2) cationic polymers or talc are thought to act as a dispersed pitch stabilizer by adsorbing on the pitch particles and rendering them non-sticky; and
(3) consecutive treatment of machine parts with cationic and nonionic polymers renders them hydrophilic by forming a steric barrier to overcome Van der Waal's attraction forces.
The third mechanism not only inhibits pitch deposition, but also gives deposits which are easy to wash away.
Surface charge neutralization of colloidal pitch in the papermaking process water suspension can be enhanced by the use of a coagulant of polydiallyldimethyl ammonium chloride (DADMAC) which has been modified to incorporate a certain degree of hydrophobic nature.
Poly(DADMAC) was found by the present inventors to be active at the pitch particle/water interface, but not active at the air/water interface. The polymer brings about coalescence of the pitch particles such that they attain a size which enables them to be caught in the matrix of the pulp fibers. The increased size facilitates them being carried along with the pulp during the papermaking process much like the process of filtration. Contact angle measurements indicate that the polymers also have a tendency to adsorb on the solid surfaces rendering them hydrophilic. The hydrophilic nature of the surface inhibits the deposition of pitch particles on them.
Based upon the inventors' investigation of poly(DADMAC) it was discovered that a cationic polymer that had the ability to get to the pitch/water interface and simultaneously adsorb on hydrophobic surfaces (like Teflon.RTM.) would be effective in controlling the deposition of pitch. Contact angle measurements showed that poly(DADMAC) was not very effective in adsorbing onto hydrophobic surfaces.
The present inventors have synthesized a novel polyelectrolyte copolymer incorporating a silicon moiety onto the backbone of a polyelectroyte such as poly(DADMAC). This unique cationic and surface active polymer is advantageous over conventional polymers especially poly(DADMAC) because of its increased surface activity, as evidenced by the lowering of the surface tension, as well as its adsorption onto hydrophobic surfaces. Both of the aforementioned properties of this novel polymer facilitate its ability to inhibit pitch deposition during the papermaking process by adsorbing on surfaces and causing the pitch particles to attain a size so that they can be carried along with the paper.
U.S. Pat. No. 5,246,547 (Finck et al.), which issued on Sep. 21, 1993, discloses a hydrophobically modified polyelectrolyte which has been used for controlling pitch. This hydrophobically modified polyelectrolyte is formed by the copolymerization of DADMAC with a hydrophobically modified monomer, such as, dimethylaminoethyl (meth)acrylate benzyl chloride quaternary, dimethylaminoethyl (meth)acrylate cetyl chloride quaternary, and dimethylaminoethyl (meth)acrylate methyl chloride quaternary.
The present inventors have uncovered that the use of a silicon moiety in place of the hydrophobically modified monomers disclosed in U.S. Pat. No. 5,246,547 results in the following advantages: (1) silicon monomers are capable of forming networks with other silicon monomers similar to crosslinking; and (2) silicon monomers are capable of adhering or adsorbing to hydrophobic surfaces even at very low levels.
The present invention also provides many additional advantages which shall become apparent as described below.